Alzheimer's disease, (AD), is the most common neurodegenerative disease in developed countries, and is characterized by progressive memory loss and impairments in language and behavior that ultimately lead to death (Alzheimer, 1911; Yankner, 1996). The cognitive decline in AD is accompanied by neuronal atrophy and loss mainly in cortex, hippocampus, and amygdala (Gomez-Isla et al., 1997). In addition to a specific pattern of neuronal cell death, AD is characterized by two neuropathological hallmarks, senile plaques and neurofibrillary tangles (NFTs).
Senile plaques are extracellular deposits of amyloid fibrils made of the 39-43 amino acid β-amyloid peptide (AB) often surrounded by dystrophic neurites (Glenner and Wong, 1984; Masters et al., 1985; Selkoe, 1994).
NFTs are intraneuronally generated aggregates of paired helical filaments (PHFs) which are assembled from hyperphosphorylated forms of the microtubule-associated protein tau (Greenberg et al., 1992; Grundke-Iqbal et al., 1986; Lee et al., 1991; Morishima-Kawashima et al., 1995). NFTs can be found in all brain regions undergoing degeneration in AD and their spatio-temporal pattern of appearance correlates well with that of cell death and symptomatology (Arriagada et al., 1992; Braak and Braak, 1991; Gomez-Isla et al., 1997).
Molecular insights into AD pathogenesis have arisen from genetic studies in families affected by inherited forms of AD (FAD). These account for only a small percentage of AD cases but have allowed the identification of mutations in three different genes that are responsible for triggering the disease. These genes are the presenilins-1 and -2 (PS-1 and PS-2) and the amyloid precursor protein (APP) (Hardy, 1996). Mutations in APP result in increased production of Aβ (Price and Sisodia, 1998) while PS-1 and PS-2 mutations favor processing of APP into the long and most amyloidogenic form of Aβ (Aβ42) (Citron et al., 1997; Duff et al., 1996; Price and Sisodia, 1998; Scheuner et al., 1996). This genetic evidence together with in vitro and in vivo studies of Aβ induced neurotoxicity point to Aβ formation and/or aggregation as a key event in triggering AD.
Little is known about downstream intracellular effectors that account for neuronal dysfunction, although activation of glycogen synthase kinase-3β (GSK-3β) has been proposed.
GSK-3β is a proline directed serine/threonine kinase that was originally identified due to its role in glycogen metabolism regulation and that is most abundant in the CNS (Woodgett, 1990). Apart from being implicated in insulin and IGF-1 mediated signal transduction, GSK-3β is also involved in the wnt/wingless signaling pathway as the key enzyme regulating β-catenin stability and, as a consequence, its translocation to the nucleus and its transcriptional activity (Anderton, 1999; Earth et al., 1997).
GSK-3β is one of the best candidate enzymes for generating the hyperphosphorylated tau that is characteristic of PHFs (Lovestone and Reynolds, 1997). GSK-3β can be purified from microtubules (Ishiguro et al., 1988) and has been shown to phosphorylate tau in most sites hyperphosphorylated in PHFs both in transfected cells (Lovestone et al., 1994) and in vivo (Hong et al., 1997; Munoz-Montano et al., 1997). Furthermore, GSK-3β accumulates in the cytoplasm of pretangle neurons and its distribution in brains staged for AD neurofibrillary changes is coincident with the sequence of development of these changes (Pei et al., 1999; Shiurba et al., 1996).
Exposure of cortical and hippocampal primary neuronal cultures to Aβ has been shown to induce activation of GSK-3β (Takashima et al., 1996), tau hyperphosphorylation (Busciglio et al., 1995; Ferreira et al., 1997; Takashima et al., 1998), and cell death (Busciglio et al., 1995; Estus et al., 1997; Forloni et al., 1993; Loo et al., 1993; Pike et al., 1991; Takashima et al., 1993). Blockade of GSK-3β expression or activity, either by antisense oligonucleotides or by lithium, prevents Aβ induced neurodegeneration of cortical and hippocampal primary cultures (Alvarez et al., 1999; Takashima et al., 1993).
PS-1 has been shown to directly bind GSK-3β and tau in coimmunoprecipitation experiments from human brain samples (Takashima et al., 1998). Thus, the ability of PS-1 to bring GSK-3β and tau into close proximity suggests that PS-1 may regulate phosphorylation of tau by GSK-3β. Mutant forms of PS-1 in transfection experiments result in increased PS-1/GSK-3β association and increased phosphorylation of tau (Takashima et al., 1998). Furthermore, PS-1 has also been shown to form a complex with the GSK-3β substrate β-catenin in transfected cells (Murayama et al., 1998; Yu et f al., 1998) and in vivo Yu et al., 1998; Zhang et al., 1998) and this interaction increases β-catenin stability (Zhang et al., 1998). Pathogenic PS-1 mutations reduce the ability of PS-1 to stabilize β-catenin, which in turn results in decreased β-catenin levels in AD patients with PS-1 mutations (Zhang et al., 1998).